Copper in various forms has long been applied to green plants as a fungicide. However, the more soluble forms of copper, such as organic copper compounds, e.g., copper sulfate, exhibit phytotoxicity to plants. Attempts to reduce the level of phytotoxicity of copper treatments have resulted in the predominate use of fixed coppers, such as copper hydroxide and copper oxychloride.
Fixed copper compounds are commonly used on potatoes during the growing season for protection against early blight and late blight diseases. At the end of the growth stage of potatoes and maturity of the tubers, the presence of green plant tissue can be a source of inoculum for the late blight disease organism. The organism can then infect the tubers during harvest, leading to increased spoilage during storage. The disease organism can also overwinter on seed tubers causing infection on plants during the subsequent growing season.
To reduce the possibility of late blight infection of tubers, the green tissue of the mature potato vine is killed and desiccated by application of a herbicide prior to harvest. However, only a few herbicides are suitable for this use. One or more fungicides can also be added to the herbicide treatment to kill the blight organisms. Copper sulfate is often used with a herbicide for this purpose since it is a relatively inexpensive form of copper and phytotoxicity is not a concern. If copper sulfate alone is used to treat potato vines, disease control would be adequate but the level of phytotoxicity alone would not be sufficient to reduce the presence of green tissue to an acceptable level.
Therefore, a need exists for a composition which will provide both sufficient desiccation of the potato vine and disease control against late blight with relatively low levels of toxicity to humans and animals.